Photocatalytic activity and photocorrosion of atomic layer deposited ZnO ultrathin films for the degradation of methylene blue

Dipole-Moment Induced Phototaxis and Fuel-Free Propulsion of ZnO/Pt Janus Micromotors

Light-driven micromotors have stimulated considerate interests due to their potentials in biomedicine, environmental remediation, or serving as the model system for non-equilibrium physics of active matter. Simultaneous control over the motion direction and speed of micro/nanomotors is crucial for their functionality but still difficult since Brownian motion always randomizes the orientations. Here, a highly efficient light-driven ZnO/Pt Janus micromotor capable of aligning itself to illumination direction and exhibiting negative phototaxis at high speeds (up to 32 µm s^-1 ) without the addition of any chemical fuels is developed. A light-triggered self-built electric field parallel to the light illumination exists due to asymmetrical surface chemical reactions induced by the limited penetration depth of light along the illumination. The phototactic motion of the motor is achieved through electrophoretic rotation induced by the asymmetrical distribution of zeta potential on the two hemispheres of the Janus micromotor, into alignment with the electric field. Notably, similar phototactic propulsion is also achieved on TiO₂ /Pt and CdS/Pt micromotors, which presents explicit proof of extending the mechanism of dipole-moment induced phototactic propulsion in other light-driven Janus micromotors. Finally, active transportation of yeast cells are achieved by the motor, proving its capability in performing complex tasks.

Photo-induced dissolution of Bi2O3 during photocatalysis reactions: Mechanisms and inhibition method

Photo-induced dissolution greatly limits the application of Bi₂O₃ photocatalyst in water treatment. In this study, mechanisms for the photo-induced dissolution of Bi₂O₃ were proposed. (1) Under UV light, h⁺ forms and diffuses through Bi₂O₃. (2) The h⁺, which reaches the surface of Bi₂O₃ and can be regarded as a monatomic oxygen ion (O_S^-), is weakly bonded to the crystal lattice. (3) Two O_S^- combine and the generated (O-O)^2- ionic group is oxidized by h⁺, resulting in the release of O₂ and dissolution of Bi₂O₃. However, modification of Bi₂O₃ using polyaniline (PANI) greatly inhibits Bi₂O₃ dissolution under UV. Under the PANI to Bi₂O₃ mass ratio of 1.5%, the concentration of produced Bi³⁺ significantly decreased from 2.02 to 0.27 mg/m² with a high methylene blue (MB) degradation efficiency of 98.3%, thanks to the separation of h⁺ from VB-Bi₂O₃ to HOMO-PANI. This study provided the theoretical foundation for the modification and application of Bi₂O₃ in water treatment.

Enhanced visible light photocatalytic activity of Fe2O3 modified TiO2 prepared by atomic layer deposition

In this work, commercial anatase TiO2 powders were modified using ultrathin Fe2O3 layer by atomic layer deposition (ALD). The ultrathin Fe2O3 coating having small bandgap of 2.20 eV can increase the visible light absorption of TiO2 supports, at the meantime, Fe2O3/TiO2 heterojunction can effectively improve the lifetime of photogenerated electron-hole pairs. Results of ALD Fe2O3 modified TiO2 catalyst, therefore, showed great visible light driven catalytic degradation of methyl orange compared to pristine TiO2. A 400 cycles of ALD Fe2O3 (~ 2.6 nm) coated TiO2 powders exhibit the highest degradation efficiency of 97.4% in 90 min, much higher than pristine TiO2 powders of only 12.5%. Moreover, an ultrathin ALD Al2O3 (~ 2 nm) was able to improve the stability of Fe2O3-TiO2 catalyst. These results demonstrate that ALD surface modification with ultrathin coating is an extremely powerful route for the applications in constructing efficient and stable photocatalysts.

A highly efficient nano-graphite-doped TiO2 photocatalyst with a unique sea-island structure for visible-light degradation

A nano-graphite-doped TiO₂ composite, C-TiO₂, was fabricated by atomic layer deposition (ALD) of TiO₂ onto carbon fiber fabrics (CFFs), followed by calcination.

Core-shell nanowire arrays based on ZnO and CuxO for water stable photocatalysts

Staggered gap radial heterojunctions based on ZnO-Cu_xO core-shell nanowires are used as water stable photocatalysts to harvest solar energy for pollutants removal. ZnO nanowires with a wurtzite crystalline structure and a band gap of approximately 3.3 eV are obtained by thermal oxidation in air. These are covered with an amorphous Cu_xO layer having a band gap of 1.74 eV and subsequently form core-shell heterojunctions. The electrical characterization of the ZnO pristine and ZnO-Cu_xO core-shell nanowires emphasizes the charge transfer phenomena at the junction and at the interface between the nanowires and water based solutions. The methylene blue degradation mechanism is discussed taking into consideration the dissolution of ZnO in water based solutions for ZnO nanowires and ZnO-Cu_xO core-shell nanowires with different shell thicknesses. An optimum thickness of the Cu_xO layer is used to obtain water stable photocatalysts, where the ZnO-Cu_xO radial heterojunction enhances the separation and transport of the photogenerated charge carriers when irradiating with UV-light, leading to swift pollutant degradation.

UV-Improved Removal of Chloride Ions from Strongly Acidic Wastewater Using Bi2O3: Efficiency Enhancement and Mechanisms

Strongly acidic wastewater generated from nonferrous metal smelting industries can be recycled as sulfuric acid after the contaminants have been removed, and among which, Cl^- is rather difficult to remove. Although previous studies showed that Cl^- can be removed from acidic Zn electrolyte by Bi₂O₃, this method still suffers from low efficiency when being employed for strongly acidic wastewater recycling. Otherwise, very high Bi₂O₃ dosage and H₂SO₄ concentration are required, leading to the need for improvement. In this study, UV irradiation was employed to improve the removal, and it was found that Cl^- removal efficiency was substantially enhanced from 63.9 to 98.3%, the optimum Bi₂O₃/Cl^- mole ratio was lowered from 1.5:1 to 0.5:1, and to achieve the maximum removal efficiency, the required H₂SO₄ concentration was lowered from 70 to 40 g/L. The mechanisms were also elaborated. First, Bi₂O₃ dissolves under the function of UV and H⁺, and the produced Bi³⁺ combines with H₂O and Cl^- to form BiOCl. Then, Bi₂O₃/BiOCl transforms into BiOCl(h⁺)/Bi₂O₃(e^-) under UV irradiation, and the generated h⁺ oxidizes Cl^- to Cl^•. Finally, Cl^• reacts with Bi₂O₃/e^- to produce BiOCl. This study offered a theoretical foundation for the improvement of Cl^- removal from strongly acidic wastewater.

Atomic layer deposition of ZnO/TiO2 nanolaminates as ultra-long life anode material for lithium-ion batteries

In this work, we designed ZnO/TiO2 nanolaminates by atomic layer deposition (ALD) as anode material for lithium ion batteries. ZnO/TiO2 nanolaminates were fabricated on copper foil by depositing unit of 26 cycles ZnO/26 cycles TiO2 repeatedly using ALD. ZnO/TiO2 nanolaminates are much more stable than pristine ZnO films during electrochemical cycling process. Therefore, ZnO/TiO2 nanolaminates exhibit excellent lithium storage performance with an improved cycling performance and superior rate capability compared to pristine ZnO films. Moreover, coulombic efficiency (CE) of ZnO/TiO2 nanolaminates is above 99%, which is much higher than the value of pristine ZnO films. Excellent ultralong-life performance is gained for ZnO/TiO2 nanolaminates, retaining a reversible capacity of ~667 mAh g-1 within cut-off voltage of 0.05-2.5 V after 1200 cycles of charge-discharge at 500 mA g-1. Constructing nanolaminates structures via ALD might open up new opportunities for improving the performance of anode materials with large volume expansion in lithium ion batteries.

TiOxNy Modified TiO2 Powders Prepared by Plasma Enhanced Atomic Layer Deposition for Highly Visible Light Photocatalysis

In this work, TiN film deposited by plasma enhanced atomic layer deposition (PEALD) is adopted to modify the commercial anatase TiO₂ powders. A series of analyses indicate that the surface modification of 20, 50 and 100 cycles of TiN by PEALD does not change the morphology, crystal size, lattice parameters, and surface area of TiO₂ nano powders, but forms an ultrathin amorphous layer of nitrogen doped TiO₂ (TiO_xN_y) on the powder surfaces. This ultrathin TiO_xN_y can facilitate the absorption of TiO₂ in visible light spectrum. As a result, TiO_xN_y coated TiO₂ powders exhibit excellent photocatalytic degradation towards methyl orange under the visible light with good photocatalytic stability compared to pristine TiO₂ powders. TiO_xN_y (100 cycles PEALD TiN) coated TiO₂ powders exhibit the excellent photocatalytic activity with the degradation efficiency of 96.5% in 2 hours, much higher than that of pristine TiO₂ powder of only 4.4%. These results clearly demonstrate that only an ultrathin surface modification layer can dramatically improve the visible light photocatalytic activity of commercial TiO₂ powders. Therefore, this surface modification using ALD is an extremely promising route to prepare visible light active photocatalysts.

Fabrication and Characterization of ZnO Nano-Clips by the Polyol-Mediated Process

ZnO nano-clips with better monodispersion were prepared successfully using zinc acetate hydrate (Zn(OAc)₂·nH₂O) as Zn source and ethylene glycol (EG) as solvent by a simple solution-based route-polyol process. The effect of solution concentration on the formation of ZnO nano-clips has been investigated deeply. We first prove that the 0.01 M Zn(OAc)₂·nH₂O can react with EG without added water or alkaline, producing ZnO nano-clips with polycrystalline wurtzite structure at 170 °C. As-synthesized ZnO nano-clips contain a lot of aggregated nanocrystals (~ 5 to 15 nm) with high specific surface area of 88 m²/g. The shapes of ZnO nano-clips basically keep constant with improved crystallinity after annealing at 400-600 °C. The lower solution concentration and slight amount of H₂O play a decisive role in ZnO nano-clip formation. When the solution concentration is ≤ 0.0125 M, the complexing and polymerization reactions between Zn(OAc)₂·nH₂O and EG predominate, mainly elaborating ZnO nano-clips. When the solution concentration is ≥ 0.015 M, the alcoholysis and polycondensation reactions of Zn(OAc)₂·nH₂O and EG become dominant, leading to ZnO particle formation with spherical and elliptical shapes. The possible growth mechanism based on a competition between complexing and alcoholysis of Zn(OAc)₂·nH₂O and EG has been proposed.

Photocatalytic Properties of Co3O4-Coated TiO2 Powders Prepared by Plasma-Enhanced Atomic Layer Deposition

Co₃O₄-coated commercial TiO₂ powders (P25) p-n junction photocatalysts were prepared by plasma-enhanced atomic layer deposition (PEALD) technique. The structure, morphology, bandgap, and photocatalytic properties under ultraviolet light were investigated systematically. Although the deposition of Co₃O₄ does not change the anatase structure and crystallite size of P25 powders, the ultraviolet photocatalytic activity has been improved evidently. For the Co₃O₄-coated P25 powders, the trace Co ions exist as Co₃O₄ nanoparticles attached to TiO₂ powder surface instead of the occupation of Ti⁴⁺ position in TiO₂ lattice. The Co₃O₄-coated P25 powders exhibit enhanced photocatalytic degradation efficiency of almost 100% for methylene blue in 1.5 h under ultraviolet light, compared with P25 of 80%. The Mott-Schottky plots of photocatalyst powders confirm the p-n heterojunction formation in Co₃O₄-TiO₂ nanocomposite materials, which is beneficial to increase the efficiency of photogenerated electron-hole separation. In addition, the Co₃O₄ coating also promotes the adsorption of organic dyes of methylene blue on P25 powders.

Novel synthesis of ZnO/PMMA nanocomposites for photocatalytic applications

The incorporation of nanostructured photocatalysts in polymers is a strategic way to obtain novel water purification systems. This approach takes the advantages of: (1) the presence of nanostructured photocatalyst; (2) the flexibility of polymer; (3) the immobilization of photocatalyst, that avoids the recovery of the nanoparticles after the water treatment. Here we present ZnO-polymer nanocomposites with high photocatalytic performance and stability. Poly (methyl methacrylate) (PMMA) powders were coated with a thin layer of ZnO (80 nm thick) by atomic layer deposition at low temperature (80 °C). Then the method of sonication and solution casting was performed so to obtain the ZnO/PMMA nanocomposites. A complete morphological, structural, and chemical characterization was made by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) analyses. The remarkable photocatalytic efficiency of the nanocomposites was demonstrated by the degradation of methylene blue (MB) dye and phenol in aqueous solution under UV light irradiation. The composites also resulted reusable and stable, since they maintained an unmodified photo-activity after several MB discoloration runs. Thus, these results demonstrate that the proposed ZnO/PMMA nanocomposite is a promising candidate for photocatalytic applications and, in particular, for novel water treatment.

Synthesis of Vertically-Aligned Zinc Oxide Nanowires and Their Application as a Photocatalyst

Vertically aligned zinc oxide (ZnO) nanowires were hydrothermally synthesized on a glass substrate with the assistance of a pre-coated ZnO seeding layer. The crystalline structure, morphology and transmission spectrum of the as-synthesized sample were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), and ultraviolet-visible (UV-Vis) spectrophotometry, respectively, indicating a wurzite ZnO material of approximately 100 nm wire diameter and absorbance at 425 nm and lower wavelengths. The photocatalytic activity of the sample was tested via the degradation of methyl orange in aqueous solution under UV-A irradiation. The synthesized nanowires showed a high photocatalytic activity, which increased up to 90% degradation in 2 h as pH was increased to 12. It was shown that the photocatalytic activity of the nanowires was proportional to the length to diameter ratio of the nanowires, which was in turn controlled by the growth time and grain size of the seed layer. Estimates suggest that diffusion into the regions between nanowires may be significantly hindered. Finally, the reusability of the prepared ZnO nanowire samples was also investigated, with results showing that the nanowires still showed 97% of its original photoactivity after ten cycles of use.

Photochemical Reaction Patterns on Heterostructures of ZnO on Periodically Poled Lithium Niobate

The internal electric field in LiNbO₃ provides a driving force for heterogeneous photocatalytic reactions, where photoexcited holes or electrons can participate in redox reactions on positive (+c) and negative (-c) domain surfaces and at the domain boundaries. One method to characterize the surface chemical reactivity is to measure photoinduced Ag deposition by immersing the LiNbO₃ in an aqueous AgNO₃ solution and illuminating with above bandgap light. Reduction of Ag⁺ ions leads to the formation of Ag nanoparticles at the surface, and a high density of Ag nanoparticles indicates enhanced surface photochemical reactions. In this study, an n-type semiconducting ZnO layer is deposited on periodically poled LiNbO₃ (PPLN) to modulate the surface electronic properties and impact the surface redox reactions. After plasma enhanced atomic layer deposition (PEALD) of 1, 2, 4, and 10 nm ZnO thin films on PPLN substrates, the substrates were immersed in aqueous AgNO₃ and illuminated with above band gap UV light. The Ag nanoparticle density increased for 1 and 2 nm ZnO/PPLN heterostructures, indicating an enhanced electron density at the ZnO/PPLN surface. However, increasing the ZnO thickness beyond 2 nm resulted in a decrease in the Ag nanoparticle density. The increase in nanoparticle density is related to the photoexcited charge density at the ZnO/PPLN interface and the presence of a weakly adsorbed Stern layer at the ZnO surface. The decrease in the nanoparticle density for thicker ZnO is attributed to photoexcited electron screening in the ZnO layer that suppresses electron flow from the LiNbO₃ to ZnO surface.

Photoactivity of hierarchically nanostructured ZnO–PES fibre mats for water treatments

Brush-like ZnO nanorods shell grown by CBD onto electrospun PES fibres as photocatalytic membranes for water purification.

Potential continuous removal of toluene by ZnO nanorods grown on permeable alumina tube filters

Vertical ZnO nanorods were successfully grown on the crystalline surface of an Al₂O₃ microfilter by the simple technique of evaporation of prepared solution at atmospheric pressure (ESAP) for photocatalytic degradation of toluene.